The Open Circulatory System is a circulatory system in which the waste products and the nutrients are carried away from one organ to another in the body with the help of the fluid. This fluids flow freely in the body cavity, instead of being contained in the veins and arteries. Example is the insect and the shellfish; these invertebrates have an open circulatory system. The fluid in them is having exact composition. Where in the vertebrates have the Closed Circulatory System in which the blood circulates through series of the vessels (veins and the arteries), and the interstitial fluid, which is called as the lymphatic fluid flowing in between the cells through the lymph nodes.
In the Open Circulatory System of the animals, all the internal structures and the organs are continuously found in mixture of the components of lymphatic fluid and the blood. The process is same as in the closed system that is the fluid brings nutrition and often oxygen to the organs, and carries away the waste products. Here, no blood pressure can be created, as there is no vessel, instead here the blood moves due to the muscle contractions. In some of the animals, the tissue does not receive oxygen from the fluid, as the fluid is not oxygenated. In such cases the tissues receives oxygen directly from the tracheal system and gets nourished.
The fluid of an Open Circulatory System is also called as the hemolymph instead of only the term blood. It is called as the hemolymph because it is the mixture of both lymphatic fluid and the blood. In the open circulatory system the blood is not pumped out of the heart in the blood vessels, but instead it directly poured into the body cavity. The hemolymph can be seen when a shellfish is opened. In shellfish, it is pinkish to green fluid.
The Advantage of and open circulatory system is the animal gets good control over their body temperature. This is the great advantage in open circulatory system. Hence, animals of open circulatory system can quickly give away the heat from the body when is a time to live in extremely warm environment. Where as the animals of the closed circulatory system have good control over the oxygen supply to the specific tissue in need. Also the animals of closed circulatory system can filter their blood more perfectly and accurately as both the lymphatic and blood systems are working separately.
Also there is one more advantage of open circulatory system that is these animals are less affected by pressure changes. Hence, because of this the mollusks family animal lives in great depths, and still their body is prevented from compression. Also the demand for oxygen is never very urgent in the open circulatory system as the oxygen supplying is done by the tracheal system.